The condensation reaction of an olefin or a mixture of olefins over an acid catalyst to form higher molecular weight products is a widely used commercial process. This type of condensation reaction is referred to herein as an oligomerisation reaction, and the products are low molecular weight oligomers which are formed by the condensation of typically 2, 3 or 4 olefin molecules with each other. As used herein, the term ‘oligomerisation’ is used to refer to a process for the formation of oligomers. Low molecular weight olefins (such as ethylene, propene, 2-methylpropene, 1-butene and 2-butene, pentenes and hexenes) can be converted by oligomerisation over a solid phosphoric acid catalyst, an acidic ion-exchange resin, a molecular sieve or a zeolite catalyst, to a product which is comprised of oligomers and which is of value as a high-octane gasoline blending stock and as a starting material for the production of chemical intermediates and end-products. Such chemical intermediates and end-products include alcohols, detergents, esters such as plasticiser esters and synthetic lubricants, polymers of unsaturated esters such as acrylic acid. The reactions typically take place in a plurality of tubular or chamber reactors. Sulfated zirconia, liquid phosphoric acid and sulfuric acid are also known catalysts for oligomerisation.
It is well known that olefins, when in contact with oxygen, can form peroxides which can render the olefins explosive. They can also cause discolouration of the olefin and can also lead to gum formation and other fouling of equipment in which the olefin is stored and/or transported. Peroxide formation can be greater if traces of iron or copper are present such as any rust on equipment. It has therefore been standard practice to incorporate antioxidants (sometimes called inhibitors) into branched chain C5 to C13 olefin oligomers in order to inhibit the problems caused by peroxide formation. It has not been necessary to do this if the olefin is to be used without storage or is stored and transported under an inert blanket such as a nitrogen blanket.
Typical antioxidants that have been used include phenolic antioxidants such as di-tertiary-butyl hydroxy toluene or butylated hydroxy toluene (BHT). A minimum of 50 to about 150 ppm by wt of the antioxidant has been used, as is shown by commercial specifications for the olefin. We have found however that the use of amounts of antioxidants in this range has a deleterious impact on subsequent reactions in which the olefin or its derivatives are used as starting materials. For example, if the olefin is used as a feedstock for hydroformylation and particularly cobalt catalysed hydroformylation, the yields in the reaction can be relatively low if such an amount of antioxidant is present. Furthermore we have found that if the alcohol obtained in such a hydroformylation reaction is used in an esterification reaction, and in particular a titanium catalysed esterification reaction, then the cycle time for the esterification reaction is relatively long. In addition, if the alcohol is used to esterify an unsaturated acid such as acrylic acid, and the acrylate ester is then polymerised, the presence of the higher amounts of the inhibitor can cause quality inconsistencies in the polymer such as variable molecular weight distribution. This, in turn, can render the polymers unsuitable for their uses such as adhesive components or lubricant viscosity index improvers.
We have now found that adequate stability may be provided to the olefin, and this may be coupled with desirable performance in subsequent reactions in which the olefin or its derivatives is a feedstock, if the antioxidant is present in an amount from 5 to less than 50 wt ppm of the olefin. We have also developed a technique that allows amounts of antioxidant within this range to be incorporated accurately and consistently into the olefin, thus providing olefins of more predictable stability.